Digital filters are used in adaptive circuits, such as adaptive subscriber line audio-processing circuits (SLACs), which require periodic updating of the filter coefficients. For example, a SLAC may employ a filter to cancel echos, or reflected signals, which occur if there is an impedance mismatch on the transmission lines interconnecting SLACs.
An adaptive filter continuously adjusts its coefficients to optimize the transfer function it implements. Without the adaption feature, a user must calculate a few sets of filter coefficients for the most commonly-encountered subscriber line characteristics and then choose the closest set of coefficients suited to the particular line used in the application. The selected set will not provide the best echo cancellation for instance, under all circumstances, especially if line impedance varies over time. By employing adaption, a user does not have to calculate any coefficients and at the same time gets a continuously-updated echo-cancellation function which can respond to changing line characteristics.
Digitally-encoded audio signals are commonly transmitted on networks employing SLACs. Accordingly, there is a need for adaptive digital filters which provide an echo cancellation function. Another application for digital filtering is in modulator-demodulators (Modems) used for transmission of digital data over telephone lines.
Because of procedures used during initial connection of SLACs and modems, echo-cancellers are deactivated for the duration of the initial establishment of the interconnection. Furthermore, the echo canceller must be deactivated if there is a "talker" at the near end of the interconnection. The adaptive filter must be prevented from updating filter coefficients during either of these circumstances. Thus, "double talker" detection is advantageously used in conjunction with an adaptive filter.